36 to 13 pin

hello every body. I saw a mini ECU board that had 36 inputs. But only they use 13 pins of a micro controller. every 4 inputs goes to an optocoupler ic, every input has two pin at output, one of them connect to a diode package and the output of diode connect to micro, another connect to same pin in other optocoupler ic and then connect to a micro input pin. how it works?